Microlithographic projection-exposure using an electron beam is highly accurate and offers prospects of greater resolution of projected pattern features than optical microlithography. However, this technique disadvantageously exhibits low throughput. Increasing the throughput of electron-beam microlithography has become the subject of much intensive development.
Contemporary projection-exposure systems perform exposure of a wafer by, e.g., cell projection, character projection, or block exposure. For example, an entire wafer is exposed by repeated transfer-exposure of small, repetitive circuit patterns (approximately (5 .mu.m).sup.2 in area as formed on the wafer) using a single small pattern segment as an exposure unit. The pattern segment is defined on a mask that also defines other exposure units each comprising a respective pattern segment. However, such a pattern-exposure system as applied to wafer exposure required for production of semiconductor integrated circuit devices (e.g., DRAMs, etc.), tends to exhibit low throughput, no more than about 10 wafers processed per hour.
Demagnifying projection-transfer apparatus employing an electron beam have been proposed that purportedly greatly improve throughput compared to the types of projection-exposure systems summarized above. In such demagnifying projection-transfer apparatus, an electron beam is irradiated onto a mask defining a circuit pattern for an entire semiconductor chip. An image of the pattern in the irradiated area is demagnified by a two-stage projection lens and transferred to a sensitive substrate. (See, e.g., Japan Kokai Patent Document No. HEI 5-160012). However, simultaneously irradiating an entire chip region of the mask by the electron beam, which transfers the entire chip pattern in one shot, results in the pattern being transferred with poor accuracy due to aberrations.
To reduce aberrations, a circuit pattern on the mask can be divided ("segmented") into multiple field segments that, in turn, are further divided into subfields. The subfields are individually transferred from the mask to the substrate in a sequential or other ordered manner. During exposure of each subfield, the performance of the electron-optical system is changed for each subfield as required to reduce aberrations. The images of the mask subfields (i.e., "transfer subfields") on the substrate are aligned and joined ("stitched") together. Reference is made to, e.g., U.S. Pat. No. 5,260,151, incorporated herein by reference.
Off-axis aberrations can be minimized in segmented projection-transfer electron-beam exposure apparatus by using axis-shifting electromagnetic lenses, such as MOL and/or VAL lenses ("MOL" denotes a Moving Objective Lens, Ohiwa et al., Electron Commun. Jpn. 54-B:44 (1971), and "VAL" denotes a Variable Axis Lens, Pfeiffer, et al., Appl. Phys. Lett. 39(9):1 (Nov., 1981)).
Third-order or three-dimensional geometric optical aberrations can be canceled by providing multiple deflectors in the projection optical system, Hosokawa, Optik 56(1):21-30 (1980).
According to conventional methods, electron-optical systems are designed by estimating various parametric values (e.g., mask-to-substrate distance, main-field dimensions, and subfield dimensions) and calculating, by simulations, the aberrations and space-charge effects impressed on the main field overall, then determining whether the specifications are met by the simulation. This has proved to be impractical and has achieved unsatisfactory results. For example, with respect to the electron lenses and/or mask-to-substrate distances, it is necessary to perform calculations to eliminate (or diminish) off-axis aberrations by means of deflectors. This requires a large amount of design time. That is, conventionally, specifications, e.g., beam current, field segment size, subfield size, etc. are defined prior to exposure. The specific design parameters, such as mask-to-wafer distance and beam angle, are set based on the defined specifications. Exposure is performed using the above design parameters and resulting aberrations are measured. If there are no significant aberrations in the resulting image, the design parameters are set. If aberrations exist, the design parameters are changed and the correction process is performed again. Since such calculations cannot be performed using all the numerous design parameters that must be considered simultaneously, the design results typically exhibit operational parameters that deviate substantially from optimal values.